Icom IC-M802 DSP, UNIT

The level-shifted signal is applied to the DSP IC (IC301) for the digital IF filter, demodulator, automatic notch and noise reduction, etc.

The output signal from the DSP IC is applied to the D/A converter section in the CODEC IC (IC501) to convert into the analog audio signals. Also the signals are level-shifted 3.3 V to 5 V at the level converter section in the IC (IC501).

The level-shifted audio signals are passed through the active filter (IC701a), and then applied to the MAIN unit via J901 (pin 17) as the DRAF signal.

3-1-6 AGC CIRCUIT (DSP AND MAIN UNITS)

The AGC (Automatic Gain Control) circuit reduces IF amplifier gain and attenuates IF signal to keep the audio output at a constant level.

The receiver gain is determined by the voltage on the AGC1 line from the DSP unit. The D/A converter for the AGC (IC102) supplies control voltage to the AGC1 line and sets the receiver gain with the [RF/SQL] control.

The 3rd IF signal from the CODEC IC (IC501) is detected at the AGC detector section in the DSP IC (IC301). The output signal from the DSP IC is level-shifted at the level converter (IC101) and applied to the D/A converter (IC102). The AGC voltage is amplified at the buffer amplifier section in the IC102 and applied to the MAIN unit to control the AGC1 line.

When receiving strong signals, the detected voltage increases and the AGC1 voltage decreases. As the AGC1 voltage is used for the bias voltage of the IF amplifiers (MAIN unit; Q203, Q401, Q501), IF amplifier gain is decreased.

3-1-7 AF AMPLIFIER CIRCUIT (DSP AND MAIN UNITS)

The AF amplifier amplifies the audio signals to the suitable driving level for the speaker.

The AF signals from the DSP unit are passed through the transmit/receive switch (MAIN unit; IC1703, pins 1, 6) via the “DSPO1” signal, and then applied to the AF amplifier (MAIN unit; IC1702, pin 5) after being passed through the low-pass filter (MAIN unit; IC1702, pins 3, 1). The amplified signal passes through the the SQL gate (MAIN unit; IC1701, pins 7, 1) which is controlled by the CPU (IC3303, pin 49) via the “SQLC” signal. The signal is applied to the electronic volume IC (MAIN unit; IC1603, pin 2) which can control the volume attenuation by AFG voltage from the CPU (MAIN unit; IC3303). Beep, tone, side tone, monitor signals are applied to the the elecronic volume IC too. The signal is applied to the AF mute swtich, and then amplified at the AF power amplifier (IC1601, pin 1). The amplified signal is applied to the speaker (SP-24) after being passed through the speaker jack (J1451) via the “AFO” signal.

•DSP CIRCUIT

IC301

IC701a

IC651b/a

DSP IC

Active

DSPO1

filter

3rd IF

Differential

CODEC IC (IC501)

signals

signal

converter

DSPI1

A/D

Level

D/A

Level

converter

(12 kHz)

MAIN UNIT

DSP UNIT

• AGC CIRCUIT

From the

antenna

DSPI1 IC1002

Q501 IF

Q401 IF

Q203 IF

amp.

IC1101

DSP

AGC1

AGC

From the

UNIT

DSC antenna

AGC2

AGC

AMP.

DSPI2

IC2702

amp.

Q2502

Q2401

Q2301

Q2302

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